Helium is a well-known gas used in cryocooling systems. Helium comes in two different isotopes: helium-3 (3He) and helium-4 (4He). Because of their low boiling points (3.19 K at 1 atm for helium-3; 4.22 K at 1 atm for helium-4), the helium isotopes are well suited for a wide range of low temperature applications where gas is the desired cooling medium. Of the two helium isotopes, helium-4 is the more commonly available isotope, making it the less expensive and more attractive isotope for cryocooling systems.
However, using helium-4 has a drawback: cooling helium-4 below its Lambda point (2.17 K for 4He) results in a phase change from a classical liquid state to a “superfluid” state. Superfluids are notable for the fact that they exhibit zero viscosity and have infinite thermal conductivity. These properties of superfluid helium-4 can be undesirable for cryocooling systems for a number of reasons. For instance, most cryocooling systems have two chamber systems that require restrictions in the mass flow rate of a fluid from one chamber to another in order to create pressure differences between two different chambers. If there is no viscosity in the fluid, there is no practical way to restrict mass flow rates to create the required pressure drops. Furthermore, a helium fluid having infinite thermal conductivity means that the temperature of the volume of fluid is practically uniform, making it particularly difficult to reduce its temperature below the critical temperature (i.e., it's Lambda point).
Current cryocooling systems that use helium-4 as the cooling medium are able to achieve temperatures as low as 1.7 K by reducing the pressure of the helium-4. However, such systems have difficulty achieving such low temperatures for long periods of time (e.g., greater than 60 minutes) without inducing a phase change in the helium-4 into a super fluid. By contrast, Helium-3 does not have a known Lambda point (it is believed to be as low as 2.5 mK), and thus does not present a superfluid issue. Having such properties, helium-3 has been a popular medium for cryocooling applications that require temperatures below 1.7 K.
Unfortunately, helium-3 is very rare. Sources of natural gas that contain helium-3 are limited, and atmospheric helium migrates into space and is lost. Because helium-3 is such a limited resource and its demand for use in cryocooling applications has increased dramatically, the cost of helium-3 has also increased dramatically. Accordingly, there is a need for a cryocooling system that is capable of maintaining temperatures at or below 1.7 K in a sample chamber for an extended period of time without having to use helium-3.